Charles Vess has never won nor been nominated for a Hugo Award for Best Professional Artist. Admitedly, outside of comics his illustration portfolio is not that large compared to some. However, this year it includes the gorgeous children's book, Blueberry Girl, written by Neil Gaiman, and next year will be added another book by Gaiman, Instructions. You can see illustrations in progress for Instructions on Vess's blog. The illustrations of both will be "whimsical, gentle and beautiful" (as April Gutierrez described Vess' artwork for Blueberry Girl here).
Blueberry Girl is really only made into fantasy through the illustrations by Vess, whereas Illustrations, with its giants, witches, and dragons, is undoubtably so. I will certainly be keeping Vess in mind as I consider the possible candidates in the next couple of years.
(I had this come to the forefront of my thoughts today as I have just been expanding the still unfinished SF Artists page for Vess.)